1. Field of the Invention
The present invention relates to the use of redundant memory cells to control self refresh activation in lower power DRAMs. More specifically, the present invention utilizes sense amplifiers attached to the redundant memory cells to determine whether an associated word line should be refreshed, thus saving power.
2. Description of the Related Art
DRAM is a volatile form of memory, which means that it must have power in order to retain data. With DRAM type memories, every couple of milliseconds the charge in the memory cells starts to disappear. This means DRAMs must be continuously and completely refreshed. Refresh is the process of recharging, or re-energizing, the cells in a memory chip. Cells are refreshed one row at a time (usually one row per refresh cycle).
FIG. 1 illustrates the standard sensing scheme of a low-power DRAM according to the prior art. First, a word line is activated, which is represented in FIG. 1 by the waveform WL. The waveform WL rises at this time. When the WL is completely up, the high word line causes the memory cells on that word line to be opened up. The memory cells then discharge on the corresponding bit line. After some time, the sense amplifiers are activated. After the sense amplifiers have sensed what is on the bit line, and the information has been read, it is amplified back into the memory cells. After this is done, the sense amplifier is switched off and the word line is driven back down. However, according to this arrangement, each word line must be refreshed continuously.
Continuously refreshing the memory cells consumes a lot of power, which is inconvenient in portable devices such as lap tops, PDAs, etc. This added power consumption speeds up the depletion of the remaining battery power in these devices.
Previously, various measures have been taken to attempt to reduce the stand-by power consumption resulting from the continuous refreshing operations. For example, architectural measures, such as attempting to shorten bit lines over the word lines in the memory array, are known in the art. However, this negatively affects the chip area because it requires additional sense amplifiers, which are comparatively large.
Another known method involves deactivating complete blocks of the memory so that unused blocks of memory do not need to be refreshed. However, the controller in this case must know which blocks are being used and which blocks are inactive and this is generally only done for very large blocks of memory.